In carrier aggregation (CA), a secondary serving cell (Scell) can be much stronger than a primary serving cell (Pcell), for example, based on reference signal received power (RSRP) measurements, due to radio resource management (RRM) inefficiencies. This is especially likely in CA scenarios with staggered beam patterns and non-overlapping coverage areas where approximately 5-10% of the UEs may have a Scell that is stronger than a Pcell by 10 dB (in RSRP) or more per R4-103677. Due to IQ imbalance, this power imbalance can result in a Scell image interfering with the Pcell signal when a wideband transceiver is used to receive the Pcell and the Scell simultaneously, e.g., two 10 MHz adjacent carriers being aggregated (intra-band CA). With an IQ gain imbalance of 1.1 (approximately 26 dB adjacent channel interference rejection ratio), a 10 dB RSRP delta between the Pcell and the Scell can lead to the Pcell signal to noise (SNR) being limited to around 15 dB thereby limiting the schedulable modulation coding scheme (MCS) on the Pcell, e.g., a modulation coding scheme (MCS) 64 QAM rate=5/6 cannot be scheduled without performance degradation, as described in R4-104310.
In LTE Rel-10, up to 5 component carriers (CCs) can be aggregated (e.g., 5×20 MHz in a 100 MHz band) in the intra-band CA case. Although the gain/phase imbalance at the local oscillator (LO)/mixer is independent of the low-noise amplifier (LNA) filter bandwidth or the analog-to-digital (ADC) bandwidth (BW) and the sampling rate, the amount of leakage of one CC into another depends on                (i) whether or not the receiver uses a single Fast Fourier Transform (FFT) or multiple FFTs, and        (ii) whether or not there are filters that follow the ADC to separate out the individual CCs in the multiple FFT case.For the simplest case of two CCs (i.e., a Pcell and one Scell), a multiple FFT receiver architecture is shown in FIG. 1 where Component Carrier #1 (CC1) is received on a lower carrier frequency relative to Component Carrier #2 (CC2). A single FFT architecture is shown in FIG. 2.        
For a receiver architecture the image from a transmission received on one CC interfering with another CC must be calibrated in CC-pairs (e.g., for 5 CCs in intra-band CA, there are
                    (                                            5                                                          2                                      )            =                                    5            !                    /                      (                                          2                !                            ⁢                              3                !                                      )                          =                  10          ⁢                                          ⁢          pairs                      )    .This entails significant effort for the User Equipment (UE) manufacturer. Further, the filter characteristics might change as a function of the number of activated/configured CCs and the bandwidth (BW) of each CC, possibly rendering pre-calibration infeasible.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.